Ovarian cancer has the highest case fatality rate of any gynecological malignancy, killing 70% of patients and accounting for almost 16,000 deaths in the U.S. annually. While most women initially respond to platin-based chemotherapy, the high relapse rate and poor response to subsequent therapy highlight the need for more effective therapy for platin-resistant disease. Our previous studies examining the action of topotecan (TPT), a topoisomerase I (topo l)-directed agent that is approved for the second-line treatment of ovarian cancer, have demonstrated that TPT induces replication fork stalling followed by activation of a kinase cascade involving ATR and checkpoint kinase 1. Building on reports that poly(ADP-ribose) polymerase-1 (PARP1) is involved in restarting stalled replication forks as well as reversing trapped topo I- DNA complexes, we have more recently demonstrated that the PARP inhibitor ABT-888 enhances TPT cytotoxicity in ovarian cancer cell lines. Even though this sensitization is more prominent in BRCA1/2- deficient cells, it also occurs in ovarian cancer cells with wildtype BRCAI and BRCA2. Moreover, this sensitization occurs at ABT-888 concentrations that are 20-fold lower than those required to kill BRCAI/2- deficient cells directly. Additional results indicate that ABT-888 is sensitizing cells through a base excision repair pathway that involves XRCC1 and suggest that PARP1 must be present for this sensitization. To further study this interaction between TPT and ABT-888, as well as extend the potential benefits of PARP inhibitor therapy to as large a subset of ovarian cancer patients as possible, we now propose to: Aim 1: Identify the mechanism by which PARP1 inhibition enhances the antiproliferative effects of TPT In ovarian cancer cells by further elucidating the DNA repair pathway that is critical for ABT- 888 mediated sensitization and determining how inhibited PARP1 actively sensitizes cells to TPT. Aim 2: Determine the mechanism of ABT-888 Induced antiproliferative effects in BRCA1/2-deflcient cells by identifying the endogenous DNA lesions that contribute to the cytotoxicity of ABT-888 as well as examining how PARP inhibition contributes to this demise. Aim 3: Evaluate the ability of a series of markers to predict response to the TPT/ABT-888 combinat ion in a CTEP-sponsored phase II trial In patients with relapsed ovarian cancer. Samples from patients enrolled on this trial will provide a unique opportunity to search for potential predictive markers of response to this regimen. These studies, which make extensive use of the Biospecimens/Patient Registry and Biostatistics Cores of the Mayo Ovarian SPORE, are designed to increase understanding of the action of the PARP inhibitor ABT- 888, both alone and in combination with TPT, in ovarian cancer, thereby advancing clinical development of the PARP inhibitor as a potentially promising ovarian cancer therapeutic. RELEVANCE (See instructions): Ovarian cancer is a lethal malignancy that claims 16,000 lives annually in the United States. ABT-888 is an investigational drug that enhances the ability of topotecan, an FDA-approved treatment for ovarian cancer, to kill ovarian cancer cells in the laboratory. The present studies are designed to not only increase understanding of the mechanism by which ABT-888 exerts its effects, but also identify ovarian cancer patients who are most likely to respond to the topotecan/ABT-888 combination in the clinic.